1. Field of the Invention
The present invention is concerned with a process for preparing polyolefins using an improved polymerization catalyst. More particularly, it relates to a process for preparing polyolefins by polymerizing or copolymerizing olefins using an organometallic compound and a catalyst comprising magnesium, titanium and aluminum or magnesium, vanadium and aluminum, the catalyst being prepared from a magnesium dihalide, an aluminum compound represented by the formula Al(OR).sub.3 wherein R is a hydrocarbon radical all three of which may or may not be identical in a particular compound and a divalent or trivalent vanadium compound or a divalent, trivalent or tetravalent titanium compound. The preferred organometallic compounds are organoaluminum, and organozinc compounds.
2. Description of the Prior Art
In the technical field of this sort there has heretofore been known a catalyst comprising a transition metal such as titanium and vanadium supported on a magnesium dihalide (see Japanese Pat. Publication No. 12105/1964). Further, in Belgian Pat. No. 742112 there is referred to a catalyst obtained by activating an anhydrous magnesium dihalide by means of a ball mill and making a titanium tetrachloride supported thereon.
In the preparation of polyolefins, however, it is desirable that the catalyst activity be as high as possible. From this point of view, with the process of Japanese Pat. Publication No. 12105/1964, the polymerization activity is still low, while, with the process of Belgian Pat. No. 742112, a considerably higher polymerization activity is attained but still desired to be improved.
In the case of German Pat. No. 2137872, a substantial reduction in the amount of MgCl.sub.2 used is attained by copulverizing a magnesium dihalide, TiCl.sub.4 and Al.sub.2 O.sub.3, but there is recognized no remarkable increase in activity per solid, a barometer of productivity, and thus a catalyst of a higher activity is still desired.
In Japanese Patent Public Disclosure No. 21777/1973 it is described that the activity is improved by the copulverization of a magnesium dihalide, TiCl.sub.4 and an aluminum halide-ether complex. Such a method, however, is disadvantageous in that the said three components all are halogen-containing substances and their halogen content is relatively high.
There has been known an example in which a magnesium dihalide and titanium trichloride were ball-milled (see Japanese Pat. Publication No. 46269/1972). It is also known from Japanese Pat. Publication No. 19885/1972 to ball-mill magnesium hydroxychloride and titanium trichloride.
In the preparation of polyolefins, however, the catalyst activity is desired to be as high as possible. From this point of view the polymerization activity in the case of the methods described in Japanese Pat. Publications Nos. 46269/1972 and 19885/1972 is still unsatisfactory and desired to be improved.
Also in the physical properties of the resulting polyolefins, a further improvement is desired as in the case of catalyst activity. For example, the impact resistance of injection moldings is desired to be as high as possible.